Geotechnical Site Characterization and Bearing Capacity Assessment for a Proposed Two-Storey Building at Nembe, Bayelsa State

Abstract

Aim: This study aimed to characterize the subsurface geotechnical properties and evaluate the bearing capacity and settlement behaviour of soils for the foundation design of a proposed two-storey building at Nembe, Bayelsa State, Niger Delta, Nigeria. Study Design:  The study employed an integrated field and laboratory geotechnical investigation, combining borehole drilling, in-situ testing, and laboratory analysis of retrieved soil samples to derive design parameters for shallow foundation assessment. Place and Duration of Study: The investigation was conducted at a proposed building site in Nembe, Bayelsa State, located within the Niger Delta region of Nigeria; fieldwork and associated laboratory testing were carried out within a single site investigation campaign during the project’s foundation design phase over a period of 3 months. Methodology: Three boreholes were drilled to depths of 20.0 m using percussion auger, with Standard Penetration Tests (SPT) performed at specified depth intervals and both disturbed and undisturbed soil samples collected for laboratory testing. Index properties (natural moisture content, Atterberg limits, and grain size distribution), shear strength parameters, and compressibility characteristics were determined following relevant ASTM and BS procedures. Allowable bearing capacity was evaluated using empirical SPT-based correlations and Terzaghi bearing capacity theory, while immediate and consolidation settlements were estimated using Christian and Carrier and Skempton and Bjerrum-type relationships. Results: The subsurface stratigraphy comprises greyish poorly graded sand (sand fill) overlying soft clayey sand to silty sand (SC–SM), greyish organic clay of low plasticity (OL), and milky poorly graded sand (SP) with shell fragments, extending to 20.0 m depth. Index properties indicate average natural moisture content of about 16.0%, liquid limit between 19.1 and 20.1%, and plastic limit between 10.5 and 11.1%, while measured SPT N-values range from 12 to 20, reflecting generally medium-dense to firm conditions. Design shear strength parameters yielded an average cohesion of about 30 kPa with a friction angle of approximately 22°, and the allowable bearing capacity, computed empirically, was about 318 kPa with a corresponding total settlement expression of 0.0127T, where T represents the applied structural load; groundwater was encountered at approximately 1.7 m below ground level. Conclusion: The geotechnical characterization indicates that, despite the presence of soft clayey and organic horizons, the subsoil at the site possesses adequate bearing capacity and controlled settlement response for a properly designed shallow raft foundation system. The results highlight the importance of detailed site-specific investigations in the Niger Delta to avoid structural distress arising from inappropriate foundation design in heterogeneous, weakly consolidated deposits.